Orthopedic bone cement is currently prepared at the time of implantation by mixing together two components, one being a liquid and the other a powder. The liquid is a monomer, typically methyl methacrylate (MMA), which also contains a polymerization activator such as N,N-dimethyl-para-toluidine (a tertiary amine) and an inhibitor such as hydroquinone to prevent the monomer from spontaneously polymerizing. The powder typically consists of small spherical beads (usually about 75 .mu.m in diameter) of poly(methyl methacrylate) (PMMA) and a small amount of a polymerization initiator such as benzoyl peroxide. Different bone cements sometimes include powders or liquids containing other acrylate polymers or monomers. Such powders also commonly include up to about 12% by weight (of the total cement composition) of a radiopaque material such as barium sulfate. The radiopacifier only serves the purpose of rendering such a bone cement radiopaque, allowing a surgeon to observe by x-ray the location and distribution of cement after surgery, and does not function as a reinforcing filler.
When a surgeon mixes the powder and liquid together, the activator in the monomer solution reacts with the initiator in the powder to produce a free radical form of benzoyl peroxide which in turn reacts with the monomer to initiate the addition polymerization of the monomer.
A problem with the current system is that the powder must have air interspersed therein so that it is sufficiently fluffy to allow mixing with the liquid monomer. Unless the powder is loosely packed, effective mixing of the two components is difficult if not virtually impossible. The result of such fluffiness and the effects of mixing a finely-divided solid and liquid together is that air tends to be entrapped within the reaction mixture and remains after polymerization. The porosity caused by such entrapped air is believed to be highly detrimental to the physical and mechanical properties of bone cements and has been implicated in the failure (by loosening) of cemented prostheses.
A further concern regarding current bone cement technology is the sensitivity of cementation results to a surgeon's technique. Different results are often obtained by different surgeons using the same cement components with different mixing and delivery techniques based solely on the ability or familiarity of the surgeon with those techniques.
Patents on orthopedic bone cements disclose variations of the above-described composition and methodology, but focus essentially on ways of making the powder and liquid components and of mixing them to form cements. In Baker U.S. Pat. No. 4,554,686, powder and liquid are premixed and then frozen to halt the polymerization reaction. Lin U.S. Pat. No. 5,334,626 discloses ways of making fine powders or a range of poly(methyl methacrylate) beads which also contain dispersed barium sulfate. Arroyo U.S. Pat. No. 5,276,070 discloses alternative powders such as butyl methacrylate, and Murray U.S. Pat. No. 5,219,897 prepares powder preforms in which the powder is partially bonded but also has continuous porosity to allow methyl methacrylate monomer liquid to surround and encase the powder.
In the dental materials field, it is known to utilize two-paste systems to produce dental restoratives that contain high percentages of reinforcing fillers in crosslinked polymeric matrices. For example, in Walkowiak et al U.S. Pat. No. 4,308,190, the patentees describe a two-paste system in which one paste contains the initiator (benzoyl peroxide) and the other paste contains an amine activator or accelerator. Both pastes contain polymer beads which, being crosslinked, cannot dissolve in monomer, along with up to 80% by weight of inorganic filler particles for improved physical and mechanical properties, and a binder constituent made primarily from difunctional (crosslinking) methacrylic acid-based monomers.
Waknine U.S. Pat. No. 4,547,531 discloses a two-paste system in which one paste has an initiator (benzoyl peroxide) and the other has the accelerator (tertiary amine); however, such paste also contain difunctional (crosslinking) monomers based on methacrylic acid and 60% to 85% by weight of inorganic filler particles which are typically silanized to enhance bonding with the polymer. The difunctional monomers and the large proportion of filler particles are provided to enhance the physical and mechanical properties of the dental restorative material.